Border with gradient and radius

.box {
  position:relative;
}
.box::before {
  content:"";
  position:absolute;
  top:0;
  left:0;
  right:0;
  bottom:0;
  border-radius:50px; 
  padding:10px; 
  background:linear-gradient(45deg,red,blue); 
  -webkit-mask: 
     linear-gradient(#fff 0 0) content-box, 
     linear-gradient(#fff 0 0);
  -webkit-mask-composite: destination-out; 
  mask-composite: exclude; 
}